Carrier current telegraph system



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NO. SYSTEM N0 2 SYSTEM 4750-2295-d 4760- 425 Hi INVENTOR W A. PHE L PSATTORNEY April 25, 1944. w PHELPS 2,347,332

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N0. 2 SYSTEM INVENTOR W A. PHELPS A 7 TORNE V April 25, 1944 w, PHELPS2,347,332

CARRIER CURRENT TELEGRAPH SYSTEM Filed Aug. 28, 1942 4 Sheets-Sheet 4VOICE mm; ssuo sewn sc Am IF/L.

BASE FREQ 4MP NO./ SYSTEM AMI? NO. 2 SYSTEM 2a 4760 was +4 R/NVENTOR WA. PHELPS BVWW A 7' TORNE V Patented Apr. 25, 1944 UNITED STATES PATENTOFFICE CARRIER cURltE lii l fli EGRAPH SYSTEM Walter A. Phelpst'Madison,N. Telephone Laboratories, York, N. Y., a corporation J., assignor toBell Incorporated, New of New York Application August 28, 1942, SerialNo. 456,469

9 Claims.

This invention relates to a, carrier current telegraph signal systemtransmitting a plurality of groups of difierent carrier waves on a lineat the same time, and more particularly to an arrangement forsynchronizing th groups of carrier waves on the line so that fixed phaserelationships are established between corresponding individual carrierWaves.

In prior carrier wave telegraph systems embodying two or more identicalgroups of different carrier waves transmitted at the same time on asingle transmission line, it was found that beat notes of relativelylarge amplitudes occurred in certain of the receiving channels with aconsequent diminution of the signal-to-interference ratio. This was dueto the fact that, although the individual carrier waves of eachgroupwere synchronized with each other, the carrier waves of each groupwere not synchronized with the carrier waves of the other groups.Consequently, at a given point on the transmission line, no fixed phaserelation existed between corresponding individual carrier waves of the,several groups of carrier waves. This further caused some'of the carrierwaves to peak up relatively sharply at one or more different points'onthe line thereby overloading both the line amplifiers and loading coilsat such points. Such beat notes and peaking introduced distortion intothe system to an extent depending on the number of carrier Wavesutilized therein and thereby impaired the signal to-interference ratioof the overall system.

The present invention contemplates an arrangement for synchronizing aplurality of groups of carrier waves of different frequenciestransmitted simultaneously on the same line.

The main object of the invention is to provide fixed phase relationsbetween corresponding individual carrier waves of a plurality of groupsof carrier waves of different frequencies.

Another object is to maintain optimum phase relationships betweencorresponding individual carrier waves of a plurality of groups ofcarrier waves of difierent frequencies.

A further object is to minimize the occurrence of momentary voltage andcurrent peaks on a line transmitting a plurality of groups of differentcarrier waves.

Still another object is to maintain substantially constant a tolerablesignal-to-interference ratio in a carrier telegraph system transmittinga plurality of groups of different carrier waves at the same time.

A still further object is to prevent momentary .overloading of lineamplifiers in a carrier tele- 55 graph system utilizing a plurality ofgroups of difierent carrier waves at the same time.

In prior carrier telegraph systems of the type to which the presentinvention is especially applicable, a plurality of synchronized carrierwaves of different frequencies were generated and supplied to aplurality of groups of channels. The carrier waves of one group ofchannels were modulated in accordance with code messagesand appliedthrough suitable sending filters to the transmission line. The carrierwaves of the other group of channels were also modulated in accordancewith code messages, and then th message modulated waves were supplied toa modulator together with a wave of an independent source of oscillationof a preselected frequency. By means of such modulation the frequencyrange of the other group of carrier waves was increased, and thereafterthe waves of the increased frequencies were selectively supplied to thetransmission line. I 1

In accordance with a specific embodiment of the present invention, meansare provided to derive the frequency-shifting wave of the preselectedfrequency by combinations of wave frequencies of the original group ofchannels. This preselected wave is then modulated together with themessage modulated waves of one groupjof carrier waves so as to increasethe frequencies of this group. Corresponding individual wavesof bothgroups being transmitted at the same time on the line are now providedwith fixed phase relationships. In one aspect, three channel currents ofdifferent frequencies'are utilized to derive the preselected Wave, andthe lower sideband components of modulation are selected fortransmission on the line. In another aspect, two channel currents ofdifferent frequencies are employed to obtain the preselected wave, andthe upper side-band components of modulation are selected fortransmission on the line. In a fur-- ther aspect, a certain even orderharmonic derived from a wave having a fundamental frequency andWhoseodd; order-harmonics serve to synchronize the channel oscillatorsis the preselected wave, and the lower side-band components ofmodulation are selected'fortransmission ontheline.

The invention will be readilyi understood from the followingdescriptiontaken together with the accompanying 'drawings'in. which:

Fig. 1 is a schematic circuit diagram showing a specific embodiment ofthe invention located at a sending terminal of a carrier telegraphsystem;

Fig. 2 is a schematic circuit diagram showing the receiving terminal tobe used with Fig. 1;

Fig. 3 is a schematic circuit diagram showing another embodiment of theinvention located at the sending terminal of a telegraph system; andFig. 4 is a schematic circuit diagram showing another embodiment of theinvention located at the sending terminal of a telegraph system.

Referring to Fig. l, a plurality of sources Ill of alternating currentwaves whose frequencies are 170 cycles apart are driven by a constantspeed motor 9 and arranged to provide different frequencies such thatthe uppermost source sup plies a wave Whose frequency is 425 cycles persecond; the next lower source provides a wave having a frequency of425+l70= 595 cycles per.

second; the third lower source provides a wave having a frequency of425-1-(2Xl70) :765 cycles per second; and so on until the lowermostsource It) provides a wave whose frequency is cycles per second. A pairof conductors I 1 connects each of the sources 110 through a sendingapparatus l2 and sending filter l3 to a pair of bus-*bars. [4. Theuppermost twelve wave sources Hi, sending apparatus l2, and sendingfilters i3 constitute the. sending terminal of a No. 1 system... Thebus-bars M are also. coupled through a sending transformer and low-passit to the input of a transformer 11 embodied in a transmission line 18which also. includes a number of suitable amplifiers [9. The low-passfilter I8 is provided with a frequency range which will be subsequentlyidentified.

Apair of conductors extending from each of .the wave'sources l0 joinsthe latter to a second group of twelve individual sending apparatus I2and sending filters 1I3, each of the latter associated sending apparatusand filters being connected by the pair of conductors H to a pair ofbus-bars 26. The lowermost group of. twelve sendingapparatus l2 andsending filters l3 comprisethe sending terminal of a No. 2 system. Thepair of bus-bars 23 is also coupled through a sending transformer 21, amodulator 23, and a band-pass filter 29 in sequence to the input of theline transformer l'l. From the foregoing it is evident that theindividual sources it are connected over two parallel branches to thetransl mission line 13..

, Referring to Fig. 2., the receiving end of the transmission line!!!terminates in two parallel branches 30 and 3| arranged to include alowpass filter 32 and a band-pass filter 33, respec tively, both ofwhich correspond in frequency characteristic to the low-pass filter I6and bandpass filter 2'9 embodied in the sending terminals of the No. 1and No. 2 systems. The output of the low-pass filter'32 is supplied to apair of busbars 34' to which are-connectedin multiple the branchesindividual to the different frequencies of the waves supplied bythesending sources 13. Each of these branches embodies in sequence areceiving-filter 35 and a receiving apparatus 33. The uppermost twelvereceiving apparatus 35 and receiving filters 36 constitute the receivingterminal of the No. 1 system. To the output of the band-pass filter 33is connected the input of a modulator 31 to which is also applied anoscillator 38 which generates. an independent wave of certain frequencyto be identified hereinafter. The output of the modulator 31 is suppliedto a pair of bus-bars, 39 to which are connected in parallel thebranches individual to the. difierent frequencies of the waves suppliedby the sending sources l0, each of the latter branches embodying insequence the receiving filter 35 and receiving apparatus 33. Thelowermost twelve receiving filters 35 and receiving apparatus 36comprise the receiving terminal of the No. 2 system. The foregoingarrangement embodying the No. 1 and No. 2 systems is essentiallydisclosed in the patent of H. Nyquist, No. 1,619,085, issued March 1,1927.

In Figs. 1 and 2, the wave sources l0, sending apparatus 12, sendingmodulator 28, receiving apparatus 36, receiving modulator 31 andoscillator 38 may comprise any one of several wellknown types of whichone is illustrated in the Nyquist patent, supra. In this connection itis understood that the. modulators may comprise copper-oxide or thyriteelements, or similar nonlinear elements.

In the operation of Figs. 1 and 2, each sending apparatus l2 embodied inboth the No. 1 and No. 2 systems may be operated to modulate the wavesof individual frequencies in four different respects so as to transmitfour messages in the manner described in the Nyquist patent, supra.Thus, each wave of individual frequency may be utilized to transmiteight messages, or a total of ninety-six simultaneous messages for boththe No. 1 and No. 2 systems. It is understood that the sending apparatusI2 may be of such design as to modulate the individual carrier waves inany desired manner; and further that modulation of each: carrier wave infour different respects as above-mentioned is selected merely for thepurpose of this illustration. The message modulated carrier waves of theNo. 1 system are applied through the sending transformer I5 and sendingfilter ['6 to the transmission line It. The sending filter 16 passeswaves whose frequencies extend over a range from 425d to 2295+d, where dis one-half the band width of each of the filters l3 of the No. 1 systemand has a value less than one-half of the cycles separating theindividual waves produced by the sources 10.

.5 The message modulated carrier Waves of the No.

2 system are applied to the modulator 28 to which, at the same time, isapplied a modulation component. whose frequency is derived in apreselected manner which will be hereinafter explained. Certain of themodulation components of the increased frequencies are selectivelysupplied to the transmission line 18 through the sending filter 29 whichpasses waves whose frequencies extend over a certain frequency rangewhich will be subsequently specified.

Atthe receiving end of the transmission line 18, Fig. 2,, the incoming,waves are divided into the two branchesby the respective filters 32 and33, of which the filter 32 possesses afrequency characteristic which is'identical with that of the sending filter I6 and the filter 33 isprovided with a frequency characteristic which is equivalentv to that ofthe sending filter 29-and which will be later mentioned. From. theoutputv of the filter 32, the message modulated waves comprising thefrequency range extending from 425-d to 2295+d cycles per second aresupplied to the twelve filters 35 and receiving apparatus 36 so that thelatter are actuated to reproduce the messages that served tomodulate thesending apparatus l2 located at the sending terminal of the No. 1system. From the output of the receiving filter 33, the messagemodulated waves of the increasedfrequencies are applied to the modulator3'! whereby the frequencies of these waves; are decreased to the rangeextending from 425-01 to 2295+d and equivalent to the frequency rangesupplied to the receiving filter 32. The message modulated waves of therange 425-d to 2295+d are applied to the receiving filters and receivingapparatus 36 comprising the No. 2 system so that the latter are actuatedto reproduce the messages that served to modulate the sending apparatusI2 located at the sending terminal of the No. 2 system. The foregoingessentially describes a carrier telegraph system utilizing at least twogroups of different carrier waves as disclosed in the Nyquist patent,supra.

In accordance with the present invention, as illustrated in Fig. l, themodulation component of preselected frequency supplied to the modulator28 and employed to increase the frequencies of the message modulatedwaves in the output of the modulator 28 of the sending terminal of theNo. 2 system is derived in a manner which will now be explained. A pairof conductors 40 connected to a pair of points 4| embodied in the pairof conductors 25 carrying the 595-cycle wave from the 595-cycle source10 of the No. 1 system to 595- cycle branch of the No. 2 system suppliesthe wave of 595 cycles per second to a modulator 42 to which is alsosupplied the wave of 935 cycles per second over a pair of conductors 43connected to a pair' of points 44 embodied in the pair of conductors 25carrying the 935-cycle wave from the 935-cycle source 18 of the No. 1system to the 935-cycle branch of the No. 2 system. The output of themodulator 42, including a component having a frequency of 2465 cyclesper second, is supplied over a pair of conductors 45 to a filter 36which attenuates all modulation components except the 2465-cyclecomponent. The latter is then amplified in amplifier 4'! and supplied toa modulator 48 to which at the same time is supplied a 2295- cycle waveover a pair of conductors 49 extending from the modulator 48 to a pairof points 58 embodied in the pair of conductors 25 carrying the2295-cycle wave from the 2295-cycle source [0 of the No, 1 system to the2295-cycle branch of the No. 2 system.

- The output of the modulator 48, including a component having afrequency of 4760 cycles per second, is supplied to a filter 5| whichattenuates all modulation components except the 4760-cycle component.The latter is amplified in the amplifier 52 and supplied over a pair ofleads 53 to the modulator 28. As above mentioned, the modulationcomponents in the output of the modulator 28 are supplied to the filter29 which is provided with a frequency characteristic which attenuatesall frequencies except those extending from 4760-2295d to .4760-425-l-d.The message modulated waves at the sending terminal of the No. 2 systemare, therefore, provided with increased frequencies embracing the latterrange and superposed on the transmission line I8 together with themessage modulated waves of the No. 1 system appearing in the output ofthe sending filter It. At the receiving terminal of the transmissionline l8, Fig. 1, the receiving filter 33 is provided with a frequencycharacteristic which is identical with that of the sending filter 29.

Deriving the 4'760-cycle component from portions of certain of the wavesproduced in the sources In and unmodulated with messages serves toprovide at any point on the transmission line [8 a fixed phase relationbetween corresponding individual waves of the groups of waves suppliedthereto by both the No. 1 and No. 2 systems. In other words,

the utilization of the 4760cycle component synchronizes all the groupsof waves of both the No. 1 and No. 2 systems during their simultaneoustransmission on the transmission line I8 whereby fixed phaserelationships are established between corresponding individual waves.This tends to obviate the occurrence of undesirable beat notes inindividual receiving channels, and the peaking of some waves at the lineamplifiers and loading coils. Optimum phase relations between all wavesof both groups of waves supplied by the No. 1 and No, 2 systems to thetransmission line l8 may be obtained by use of the well-known phasecontrol networks, not shown, in the outputs of certain of the sourcesI0; approximately optimum phase relations in the foregoing respect mayalso be provided by reversing the output terminals of certain of thesources l0, which reversal is common to the art. Such optimum phaserelationships are preserved because the synchronization of correspondingindividual waves of the groups of waves in the line prevents drifts infrequency of one or more of the wave sources l0 which frequency driftswould cause desired phase relationships between individual waves to belost in a few seconds. As a consequence, distortion in the system ismaintained substantially constant at a certain tolerablesignal-to-interference ratio.

Another arrangement for establishing a fixed phase relation between thecorresponding individual waves of both the No. 1 and No. 2 systems isillustrated in Fig. 3. In the latter arrangement, conductor pairs 55 and56 supply 425- and 1615-cycles waves to a modulator 5'! whoseoutput'among other modulation products includes the component having afrequency of 2040 cycles per second. The output of the modulator 51 issupplied to a filter 58 which serves to attenuate all modulationproducts except the 2040-cycle component which is amplified in amplifier59 and supplied to the modulator 60. The output of the latter issupplied to the band-pass filter 29 which has a frequency characteristicextending from 2040+425-d to 2040+2295-l-d, or a frequencycharacteristic which is essentially the same as that of the sendingfilter 29 of Fig. 1. In other respects, the operation and advantages ofFig. 3 are identical with those of Fig. 1.

Instead of the multifrequency source In utilized in Figs. 1 and 3, agroup of phased and synchronized channel oscillators 6!), Fig. 4, may beemployed as the individual sources of different frequencies as disclosedin my copending application, No. 356,128, filed September 10, 1940. Inthis connection a source 6| supplied an alternating current wave havinga fundamental frequency of cycles per second to an amplifier 62 whoseoutput is impressed through a tuned circuit 63 onto a harmonic generatorwhich may be of the type disclosed in the patent of L. R. Wrathall, No.2,117,752, granted May 17, 1938. The output of this harmonic generatorcomprising certain odd harmonics of the fundamental frequency isimpressed on the individual channel oscillators 60. However, for thepurpose of the present invention, the 4760-cycles wave employed in themodulator 28, Figs. 1 and 4, to increase the message modulated waves ofthe No. 2 system is obtained from the output of the harmonic generator64. As the output of the latter embodies only odd harmonics of the85-cycle fundamental wave, and further as the 4760-cycle wave is the56th even harmonic of the 85-cycle fundamental wave, the output of theharmonic generator 64 is also supplied to the vertical diagonal of a,rectifier bridge 65 which translates the odd order harmonics into evenorder harmonics of the 85-cycle fundamental wave so that the even orderharmonics appear across the horizontal diagonal of rectifier bridge 65as pointed out in the Wrathall patent, supra. The even order harmoniesare supplied to a filter 85 which has such frequency characteristic asto pass the frequency of 4760 cycles per second and to attenuate theother harmonics. The 4'760-cycle harmonic is supplied through theamplifier 52 to the modulator 28. In other respects, the operation andadvantages of Fig. 4 functions are identical with those of Fig. 1.

What is claimed is:

In combination in a carrier current signal system, comprising atransmission line, means to generate a plurality of alternating currentshaving different frequencies and synchronized with respect to eachother, circuit means to provide at least two branches extending fromsaid generating means for each current of different frequency, separatemeans to modulate the currents of each of said branches for each currentof different frequency in accordance with code messages, means to applythe group of message modulated currents of one of said branches for eachcurrent of different frequency to said line, means to increase thefrequencies of the message modulated currents of the other of saidbranches for each current of different frequency and to apply a selectedgroup of currents of the increased frequencies to said line, means atthe receiving end of said line to separate the group of messagemodulated currents of said one branch for each current of difierentfrequency and the selected group of currents of the increasedfrequencies into two further branches, and means to utilize the currentsof said further two branches to reproduce the code messages; means toprovide corresponding individual currents of the groups of currentstransmitted on said line with fixed phase relationships, such that thepeak magnitudes of the corresponding currents on said line and theproduction of heat notes in said current utilizing means aresubstantially minimized, comprising means to obtain a current of certainfrequency from preselected current of said generating means, and circuitmeans to apply said certain current to said frequency increasing meansfor modulation with the group of message modulated currents of saidother branch for each current of different frequency whereby said lattermodulated currents are increased in frequencies.

2. A carrier current signal system comprising a transmission line, aplurality of sources to generate alternating current waves havingdifferent frequencies and synchronized relative to each other, circuitmeans to provide at least two branches extending from said generatingmeans for each current of different frequency, circuit means to modulatethe different currents of each of said two branches for each current ofdifferent frequency in accordance with code messages,

means to apply the group of message modulated currents of one of saidbranches for each current of different frequency to said line, means toincrease the frequencies of the message modulated currents of the otherof said branches for each current of different frequency and to apply aselected group of currents of the increased frequencies to said line,and means to provide fixed phase relationships between correspondingindividual currents of the groups of currents transmitted on said linesuch that the peak magnitudes of the corresponding individual currentson said line are maintained substantially at minimum values, comprisingmeans to derive a component of predetermined frequency from certain ofthe generated currents and unmodulated with the code messages, andcircuit means to apply said predetermined component to said frequencyincreasing means for modulation together with the message modulatedcurrents of said other branch for each current of different frequencywhereby said latter modulated currents are increased in frequencies.

3. The carrier current signal system according to claim 2 in which saidderiving means derives said component of predetermined frequency fromthree of the generated currents of different frequencies and unmodulatedwith the code messages.

4. The carrier current signal system according to claim 2 in which saidderiving means derives said component of predetermined frequency fromtwo of the generated currents of different frequencies and unmodulatedwith the code messages.

5. A carrier current telegraph system comprising a plurality ofgenerators to produce alternating currents of different frequencies andsynchronized relative to each other, circuit means to provide twobranches extending from each of said generators, separate means tomodulate the different currents of one of said branches for each currentof different frequency in accordance with code messages and to applythis group of modulated currents to said line, separate means tomodulate the different currents of the other of said branches for eachcurrent of difierent frequency in accordance with code messages, meansto increase the frequencies of the modulated currents of said otherbranch for each current of difierent frequency and to apply a selectedgroup of currents of the increased frequencies to said line, and meansto synchronize the groups of currents on said line so that fixed phaserelationships are established between corresponding individual currentsfor controlling the peak magnitudes of said latter currents on saidline, comprising means to derive from portions of three of the differentgenerated currents unmodulated with the code messages a component whosefrequency is above the frequencies of the generated currents, circuitmeans to apply said component to said frequency increasing means to bemodulated with the message modulated currents of said other branch foreach current of different frequency whereby said latter modulatedcurrents are increased in frequencies, and selective means embodied insaid frequency increasing means to apply to said line the selected groupof currents of increased frequencies comprising the lower side-bandcomponents of the modulation components in said frequency increasingmeans.

6. A carrier current telegraph system comprising a plurality ofgenerators to produce alternating currents of different frequenciessynchronized relative to each other, circuit means to provide twobranches extending from each of said generators, separate means tomodulate the difierent currents of one of said branches for each currentof different frequency in accordance with code messages and to applythis group of modulated currents to said line, separate means tomodulate the different currents of the other of said branches for eachcurrent of different frequencies in accordance with code messages, meansto increase the frequencies of the modulated currents of said otherbranch for each current of different frequency and to apply a selectedgroup of currents of increased frequencies to said line, and means tosynchro nize the groups of currents on said line so that fixed phaserelationships are established between corresponding individual currentsfor controlling the peak magnitudes of said latter currents on saidline, comprising means to derive from portions of two of the differentcurrents unmodulated with the code messages a component whose frequencyis below the highest frequency of the generated currents, circuit meansto apply said component to said frequency increasing means to bemodulated with the message modulated currents of said other branch foreach current of different frequency whereby said lattermodulated-currents are increased in frequencies, and selective meansembodied in said frequency increasing means to apply to said line theselected group of currents of increased frequencies comprising the upperside-band components of the modulation components in said frequencyincreasing means.

7. A carrier current telegraph system cornprising a plurality ofgenerators to produce alternating currents of different frequencies andsynchronized relative to each other, circuit means to provide twobranches extending from each of said generators, separate means tomodulate the different currents of one of said branches for each currentof different frequency in accordance with the code messages and to applythis group of modulated currents to said line, separate means tomodulate the different currents of the other of said branches for eachcurrent of different frequency in accordance with code messages, meansto increase the frequencies of the message modulated currents of saidother branch for each current of diffehent frequency and to apply aselected group of currents of increased frequencies to said line, andmeans to establish fixed phase relationships between cor? respondingindividual currents of the two groups of currents on said line forcontrolling the peak magnitudes of said latter currents on said line,comprising means to derive from portions of two different currentsunmodulated with the code messages a component of certain frequency,means to modulate the certain component with a portion of anotherdifferent current unmodulated with the code messages to produce acomponent of further frequency, means to apply the further component tosaid frequency increasing means for modulation with the messagemodulated currents of said other branch for each current of differentfrequency whereby said latter modulated currents are increased infrequencies, and selective means embodied in said frequency increasingmeans to apply to said line the selected group of currents of increasedfrequencies which frequencies are above the frequency range of theproduced currents.

8. A carrier current telegraph system comprising a plurality ofgenerators to produce alternating currents of different frequencies,means including a harmonic generator to apply certain odd harmonics of acurrent of base frequency to said generators to synchronize the currentsproduced thereby, circuit means to provide two branches extending fromthe output of each of said generators, separate means to modulate thedifferent currents of one of said branches extending from the output ofeach of said generators in accordance with code messages and to applythis group of modulated currents to said line, separate means tomodulate the currents of the other of said branches extending from theoutput of each of said generators in accordance with code messages,means to increase the frequencies of the message modulated currents ofsaid other branch extending from the output of each of said generatorsand to apply a selected group of currents of increased frequencies tosaid line, and means to establish certain phase relationships betweencorresponding individual currents of the two groups of currents on saidline for controlling the peak magnitudes of said latter currents on saidline, comprising means to derive a certain even order harmonic of saidbase frequency from the odd order harmonics in the output of saidharmonic generator, circuit means to apply said certain even orderharmonic to said frequency increasing means for modulation with themessage modulated currents of said other branch extending from theoutput of each of said generators whereby said latter modulated currentsare increased in frequencies, and selective means embodied in saidfrequency increasing means to apply to said line the selected group ofcurrents of increased frequencies which frequencies are above thefrequency range of the produced currents.

9. In a carrier telegraph system, a group of channels having carriefrequencies in a low range, a group of channels having carrier frequencies in a hgh range above said first range, means to generate thecarrier waves in the firstmentioned range with the desired frequencyspacing and phase relationships, means to provide similar frequencyspacings and phase relationships for the carrier waves of thesecondmentioned range comprising means to modulate waves havingfrequencies in said first range and derived from said first means, witha wave of fixed frequency and predetermined phase to shift thefrequencies of said derived waves into said high range, and means toderive said Wave of fixed frequency and predetermined phase byintermodulation between certain of the carrier waves in said firstrange.

WALTER A. PHELPS.

